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The amino-terminal sequence of two electrophoretically homogeneous forms-of rabbit liver microsomal cytochrome P-4S0, P-450KM 2 and P-450KMw, has been examined by automated Edman degradation. Methionine is the amino terminus of P-4SOLM2, and 17 of the first 20 residues are hydrophobic, including two clusters of five consecutive leucines. The composition and sequence of this region are similar to those of the short-lived hydrophobic amino-terminal precursor segments of certain other proteins, especially myeloma immunoglobulin light chains and pancreatic zymogens. Multiple amino-terminal residues, including methionine, were detected for P-4SOLM 4 suggesting the presence of several highly similar forms of P-4S0 or that partial proteolysis had occurred. During the past few years, several different forms of liver microsomal cytochrome P-450 (P-4SOLM) have been separated and purified (1-7). We have recently begun an investigation of the primary structure of P-450LM as an aid in understanding the variations in the biological and chemical properties of this family of proteins, which play an essential role in the metabolism of prostaglandins, fatty acids, and steroids as well as a host of foreign compounds, including drugs and carcinogens.